1. Field of the Invention
The present invention relates to an optical recording medium such as optical disk memory and an optical card, information being writable thereon and readable therefrom with the irradiation of a laser.
2. Description of the Prior Art
As shown in FIG. 1, a conventional optical recording medium 30 is made up of a transparent substrate 32 typically made of an acrylic substance, polycarbonate or glass having a surface on which a tracking guide groove 31 is formed, an interference layer 34 made of SiAiON or AlN, a magneto-optic recording layer 37 made of GdTbFe or TbFeCo, a protective layer 38 made of SiAiON or AlN, and a reflection layer 40 made of Al. These layers are successively formed one above the other in the stated order. Recordation of information on such an optical recording medium is carried out by irradiating a laser onto the magneto-optic recording layer 37 and heating the layer 37 above a Curie temperature or a compensation temperature while simultaneously applying an external magnetic field thereto to thereby create inverse magnetization of the layer 37. Reproduction of the information from the recording medium is achieved by utilizing the Kerr effect in which the rotation of the plane of polarization of the reflection light is reversed depending on the magnetization direction when a laser of linear polarization is irradiated onto the recording layer 37. To obtain a good signal characteristic, it has been necessary to increase a virtual Kerr rotating angle with Kerr effect enhancement and to improve a signal-to-noise ratio. To this end, the interference layer 34, having a higher refractive index than the substrate 32, is interposed between the substrate 32 and the recording layer 37 to utilize an interference effect which results when there are multiple reflections of light.
In such a conventional optical recording medium, the thickness of the interference layer 34 is set substantially to .lambda./4n in order to increase the Kerr effect enhancement resulting from the interference effect at the substrate, interference layer, recording layer, protective layer and the reflection layer, where .lambda. is a wavelength of a reproducing light and n is a refractive index of the interference layer 34. However, as can be appreciated from the curves depicted in FIG. 2, setting the thickness of the interference layer 34 to .lambda./4n adversely reduces the reflection of light from the groove 31. Further the thickness of the recording layer 37 is set so that the reflection of light from the recording layer 37 is a minimum. Therefore, the push-pull signal used for tracking servo is substantially zeroed. Consequently, the tracking servo becomes unstable and thus stable reproduced output cannot be obtained. For the reasons stated above, it is impossible to attain the maximum Kerr effect enhancement and there is the problem that an allowable range of thickness of the interference layer is so limited and thus an allowable variation in the thickness of the layer is very small.